Mechanism of Ca-Ba diffusion in lead-free (Ba,Ca)TiO 3 piezoelectrics
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Mechanism of Ca-Ba diffusion in lead-free (Ba,Ca)TiO3 piezoelectrics Chang Shu 1, Daniel Reed 1, Tim Button 1,2 1 School of Metallurgy and Materials, University of Birmingham, Birmingham, B15 2TT, UK 2 Central European Institute of Technology (CEITEC), Brno 60200, Czech Republic Abstract The reaction mechanism of BaCO3+CaCO3+TiO2 by solid state methods has been studied in this work using thermal analysis (DSC-TG) from 500 to 1500 ºC and in situ X-ray diffraction (XRD) from room temperature to 800 ºC. In the mixed powders, the CaO is firstly formed followed by presence of an intermediate Ba2TiO4 phase and finally the formation of CaTiO3, BaTiO3 and/or (Ba,Ca)TiO3, where the presence of CaO or CaTiO3 (CT) has slowed down the formation of BaTiO3 (BT). Raman microscopy of a BT-CT diffusion couple has shown that Ca2+ firstly diffuses into the BT grain boundaries and then into the BT core. 1. Introduction Piezoelectric ceramics are widely used in sensors, actuators and ultrasonic transducers due to their ability to achieve efficient conversion between electric and mechanical vibrations. There is an urgent desire to move to lead-free materials achieving comparable piezoelectric performance to lead-based materials. The pseudobinary system xBa0.7Ca0.3TiO3-(1-x)BaTi0.8Zr0.2O3 (BCZT) has been reported to achieve a piezoelectric charge coefficient (d33) of 620 pC/N when x=0.5 due to the presence of a morphotropic phase boundary [1]. BCZT can be synthesised by combining Ba0.7Ca0.3TiO3 (BCT) and BaTi0.8Zr0.2O3 (BZT) in the solid state, requiring substitution of Ca and Zr on the Ba-site and Ti-site respectively. BCT is also an important piezoelectric material (d33=180-310 pC/N) due to the structural changes caused by the substitution of Ca in BaTiO3 [2]. However, the rate of Ca substitution has been reported to be a limitation [3]. Therefore, a detailed understanding of this Ca-Ba substitution mechanism in solid state method is necessary to enable a fundamental structural study into the synthesis and properties of BCT and BCZT ceramics. A reaction mechanism for BaCO3 and TiO2 by solid state methods has been reported by Beauger et al. [4,5]. By heating a reaction couple of BaCO3/TiO2 to 750 ºC for 24 hours, they observed the presence of Ba2TiO4 phases in contact with TiO2. They also stated that the formation of Ba2TiO4 rather than BaTiO3 was caused by the rate of diffusion of Ba2+ into TiO2 being slower than the rate of arrival of Ba2+ at the TiO2. Raman spectral imaging (RSI) has been used to observe the phase composition and distribution of core-shell structured BaTiO3 precursors with a TiO2 core and a BaCO3 shell, confirming the presence of Ba2TiO4 phase at 900 ºC, located in the outermost Ba-rich region [6]. Based on Beauger’s mechanism, kinetics of BaTiO3 formation reaction could be improved by changing to a more powerful milling method to increase the Ba2+ diffusion rate into TiO2 [7,8]. By mechanical milling for 159 hours [9], the Ba2TiO4 phase was observed at 650 ºC by in situ XRD (2θ=28.5-29.5º), corresponding to a small e
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